During the process of manufacturing polycrystalline silicon cell pieces, primary metals precipitated in the silicon wafer can be easily decomposed into unstable metal composites under a high-temperature sintering treatment. These metal composites possess a certain composite activity, which is capable of seriously reducing the conversion efficiency of the polycrystalline silicon cell. During the use of the cell, metal composites can be converted into semi-stable metal composites having a high composite activity. These semi-stable metal composites can further decrease the conversion efficiency of the polycrystalline silicon cell. Even worse, metal atoms having a high composite activity can be formed during the later decomposition of the semi-stable metal composites, resulting in a low conversion efficiency of the polycrystalline silicon cell.